The isolation of vibrations transmitted to the drivers of conventional or cab-over-engine (COE) trucks has been a problem which has attracted a considerable amount of attention among vehicle OEM and component manufacturers for some time. Cabs mounted on the frame separately from the load carrying portion of a truck are subjected to vibrations from the frame, wheels and engine. While wheel vibrations into the truck frame may be reduced at least to some degree by selection of appropriate suspension springs and shock absorbers, and engine vibrations may be isolated with elastomeric mountings, the cab induced vibrations coming from the frame still pose a difficult problem as discussed in U.S. Pat. No. 3,554,596 to LeFevre.
Examples of prior art suspension systems intended to minimize the transmittal of frame vibrations to the truck cab include the rigid metal springs taught in LeFevre and the air spring system disclosed in U.S. Pat. No. 3,948,341 to Foster. Although the LeFevre and Foster suspension systems appear to provide reasonable ride quality, it is believed that superior performance can be achieved using a diaphragm type hydraulic-pneumatic suspension system such as disclosed in U.S. Pat. Nos. 3,479,053; 3,669,225; and 3,802,686 to Moulton. As discussed in detail below, the subject invention is directed to a tilt-assist mechanism adapted for use with such hydraulic-pneumatic suspension systems in a cab-over-engine type truck.
Generally, the suspension unit in a cab-over-engine truck should facilitate tilting of the cab to provide access to the engine without sacrificing the desired vibration isolation capability. It is believed that each of the suspension systems identified above would provide improved ride quality if the metal springs, air springs or hydraulic-pneumatic units were permitted to provide the only stiffness and damping responses between the cab and frame. However, to accommodate the tilting operation required in cab-over-engine trucks and to prevent cab-to-frame separation in crash situations, such elements as pivoting arms and bushings have been included in prior art designs as shown for example in the Foster patent. While not intended to have noticeable influence on the vibration isolation capability of a suspension system, pivoting arms and bushings have the effect of increasing the effective stiffness of a suspension system resulting in a significant degradation of the overall quality of the ride. Therefore, to the extent that such cab tilting assist mechanisms may be removed from cab-over-engine cab suspension systems, the expected ride quality of any system would be improved.